The present invention relates to AC generators and, more particularly, to AC generators that are driven by mechanical rotational force exerted on a shaft of the AC generator, in order to provide either a constant frequency output from a variable speed input, an output having a frequency within a narrow range from a variable speed input, or a constant frequency output that is shifted in space and time from a constant speed input.
Many aircraft include AC generator systems to supply relatively constant frequency AC power. Many of the AC generator systems installed in aircraft include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes a rotor having permanent magnets mounted thereon, and a stator having a plurality of windings. When the PMG rotor rotates, the permanent magnets induce AC currents in PMG stator windings. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current to the exciter.
The exciter typically includes single-phase (e.g., DC) stator windings and multi-phase (e.g., three-phase) rotor windings. The DC current from the regulator or control device is supplied to exciter stator windings, and as the exciter rotor rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the main generator. The main generator additionally includes a rotor and a stator having single-phase (e.g., DC) and multi-phase (e.g., three-phase) windings, respectively. The DC currents from the rectifier circuits are supplied to the rotor windings. Thus, as the main generator rotor rotates, three phases of AC current are induced in main generator stator windings. This three-phase AC current can then be provided to a load such as, for example, electrical aircraft systems.
Many of these AC generator systems are driven by variable speed prime movers. For example, many generators are driven by the aircraft engines, which may vary in rotational speed during operation. Thus, to ensure the AC generators supply relatively constant frequency AC power, many aircraft include a hydro-mechanical transmission, or other type of gear arrangement, that converts the variable engine speed to a relatively constant rotational speed.
Although the above-described configuration is generally safe, it does suffer certain drawbacks. For example, hydro-mechanical transmissions can be relatively large, heavy, and complex, and they may also exhibit relatively poor reliability. Each of these factors can lead to increased overall aircraft, fuel, and maintenance costs, and/or increased maintenance frequency, which can further lead to increased costs.
One solution to the above-noted drawbacks is disclosed in U.S. Pat. No. 6,188,204 to Vithayathil et al. The solution disclosed therein employs main windings and auxiliary windings disposed on the same rotor. The auxiliary windings are supplied with adjustable frequency AC power, and in turn excite the main windings to produce a desired three phase output frequency. Although this solution does work, it also suffers certain drawbacks, namely, the main and auxiliary windings must be arranged to be magnetically decoupled by configuring a specified numbers of poles, so that the flux generated by the main windings does not induce any voltage in the auxiliary windings that has a different number of poles or vice versa. This can lead to complexity in design and implementation.
Another solution is disclosed in U.S. Pat. No. 7,064,455 to Lando. This solution employs rotor windings of the exciter and the main generator disposed on the same primary shaft, with permanent magnet generator (PMG) disposed about an associated secondary shaft, for determining the rotational speed of the primary shaft. The rotors of the exciter and main generator employ three-phase windings. However, this design offers no improvement over the complexity inherent in such three-phase windings.
Hence, it can be seen that there is a need for a system and method of supplying relatively constant frequency AC power from a generator that is driven by a variable speed prime mover that is a relatively small, lightweight, less complex, and more reliable, as compared to current systems and methods, and that does not rely on specified numbers of exciter and main generator poles.